Digital document and image scanning devices may include a bed or other surface across which a document or other image bearing medium is passed or positioned in combination with an optical scanning device, typically a CCD or CIS device for obtaining an optical image from the document. Often times, in combination with the bed or the optical scanning device, separate method or means for detecting the presence of a document which is skewed and/or misregistered, hereinafter simply skewed, in excess of some predefined limit is employed. This feature allows the scanning device to sense the presence of a condition wherein the document is skewed relative to the direction of travel of the optical scanning device. Detecting skew is important because the document may be skewed to such an extent that data or imagery may be lost due to the fact that such data or imagery falls outside the range of the optical scanning device.
Additionally, if a document is skewed in excess of the limits of the document feeder, a paper jam can result. In order to provide for smooth and continuous operation of the document feeder, skew detection is used to detect the position of a document which is skewed in excess of some predefined limit, thus allowing the feed mechanism to be deactivated and/or reversed before a paper jam occurs. Also, optical character recognition algorithms rely on a generally horizontal base line in order to recognize letters, words and lines of text as well as other graphic elements. Skew introduces error into this process.
Currently, one means for detecting the presence of a document which is skewed in excess of some predefined limit uses a pair of marginal sensors which are separate components from the primary vision transducer, raising the cost and complexity of the device. Each sensor is positioned at a distance from the other which is approximately equal to distance between the two side margins of the document being fed through the document feeder. Operatively connected to each of the two the marginal sensors is a means for deactivating the optical scanning device and/or the feed mechanism when a portion of the document being fed enters the range of either of the marginal sensor.
In accordance with this prior art device and method, a pair of marginal sensors are employed each requiring additional electrical and mechanical connection and peripheral devices which operate in concert with the marginal sensors.